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R. Mallery, R. Rich, S. Salim (UC Los Angeles), L. Kewley (IFA Hawaii), GALEX Team
We investigate the production of nitrogen in star forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 1,080 GALEX selected emission line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. To perform the analysis, we derive both gas-phase oxygen and nitrogen abundances for the sample, and use stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. Nitrogen abundances are derived from the [NII]6584/[OII]3727 ratio using empirical calibrations to estimate the temperature of the NII ionization regions and to calculate values of [N/O]. Oxygen abundances are derived using three different methods: two based upon empirical calibrations and one utilizing models of evolving stellar populations and photoionization. We find that the methods agree for oxygen abundances, 12+log[O/H]>8.6. All three estimates show that for [N/O] >-1.4, [N/O] has a linear dependence on oxygen abundance. We find that in this abundance range, the [N/O] values lie on the linear proportional theoretical curve given by the simple closed-box model of Vila-Costas & Edmunds (1993). For [N/O] <-1.4 in two abubdance calculations, the galaxies lie between the constant and linear proportionality theoretical curves when [N/O] is plotted versus [O/H]. This may be due to primary nitrogen in massive stars or galactic winds. The galaxies have moderate to high values of derived current star formation rates in the range of .1 and 100 M\odot yr-1. We find that galaxies with values of current star formation rate divided by past time-averaged star formation,~log b <0, have slightly higher values of [N/O] than the galaxies with log b >0. This is likely due to the release of primary and secondary nitrogen by intermediate-mass stars in the galaxies with log b <0 that formed during the last burst of star formation. This result agrees with the evolutionary scenario where there is a time delay between the release of the bulk of the nitrogen and oxygen, with the bulk of the primary nitrogen being released from intermediate-mass stars.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.